1. Field of the Invention
This invention relates to improvements in gas chromatograph (GC), Fourier transform (FT) infra-red (IR) spectroscopy systems and, more particularly, to improvements in a GC/IR interface assembly which can be used in substantially any commercial FTIR sample chamber.
2. Prior Art
The combination of GC and FTIR analytical techniques is well known whereby a sample gas is separated into its constituents. An FTIR spectroscope and data processing system detects and rapidly analyzes the constituents. GC/FTIR systems generally include a GC having an oven containing a separating column. A carrier gas continuously flows through the separating column. A sample gas mixture, initially in liquid form, is injected into the GC and vaporized so that its constituent parts are "picked up" by the carrier gas and separated as the carried constituents flow through the GC separating column. The column effluent enters a light pipe through which each constituent (or peak) moves at different times. IR light (energy), provided by an interferometer, is directed through one end of the light pipe and is absorbed by the constituent parts thereby identifying the type and quantity of each constituent. The IR energy is detected at the opposite end of the light pipe such that an electrical signal or interferogram is created. The interferogram then becomes input data to a data processing system. In the data processing system, the "digitized" interferogram is subjected to Fourier analysis as well as other mathematical computations that provide the characteristics of the constituents of the sample.
In prior art GC/FTIR systems, the separating column is located within an oven and the light pipe is located external to the oven. A transfer tube is connected, at one end, to the exit end of the separating column, and at the other end, to the input end of the light pipe. U.S. Pat. No. 4,440,013, assigned to the assignee of the present invention, shows how a narrow component peak from the GC separating column can be transferred to the input end of the light pipe in order to achieve high chromatographic resolution and a high signal-to-noise ratio when using the IR detector. However, the problem remains of transferring the sample to a GC detector which is connected, via a transfer tube, to the output end of the light pipe. The type of sealants and fittings used to connect the transfer tube to the output end of the light pipe detector as well as the type of transfer tube used can cause or be responsible for turbulence and eddy currents which create dead volumes such that peak (band) broadening and tailing of the sample can occur. Peak broadening will affect, by reducing, the chromatographic resolution and the spectrometric detection of the constituents. This problem becomes more acute if an air tight seal is not provided between the transfer tubing and the output end of the light pipe. The problem is further exacerbated if the sealants that are used contaminate or absorb the sample thereby creating a source of error.
Also, in prior art GC/FTIR systems, the light pipe extends through a heat transfer block around which is wrapped a heater coil. The transfer tube(s) also extend through a second heat transfer block which is wrapped by another heater coil. During operation of the system, the various heaters are independently energized in order, for example, to prevent condensation of gas in the light pipe. However, prior art heaters implemented in this manner do not permit optimal temperature control. Also, the prior art systems, having a variety of transfer tubes, fittings, heat transfer blocks and heater coils, are difficult to manufacture and to maintain. Furthermore, prior art light pipes, associated transfer tubing, heat transfer blocks and heater coils are not easily adapted to be received by and to work with a variety of different GC/FTIR systems.
U.S. Pat. No. 4,420,690 assigned to Bio-Rad Laboratories, Inc. discloses a sample cell which includes a small-bore hollow tube through which a gas stream flows and end windows at the ends of the tube which are captivated within a thermally conducting holder. The cell also includes matching bores in the holder for accommodating fluid transfer lines which are sealed to the tube. The windows are sealed to the ends of the tube by a sealant which is exposed to the flow stream thereby exposing the sample to contamination or reaction with the sealant. The sealant is also used to seal the transfer lines to the tube thereby again exposing the sample to contamination or reaction with the sealant. The windows are held at the ends of the tube with a sealant, an elastic ring such as a rubber spring and a washer. The washer is secured by screws.
Accordingly, it is an object of this invention to provide a compact, easily removable GC/IR interface accessory containing a floating light pipe assembly, heater block and mirrors for use in substantially any FTIR spectrometer sample chamber.
Another object of this invention is to reduce the peak broadening in the transfer tube at the output end of the light pipe in order to achieve high resolution in the GC detector.
Another object of the invention is to provide an air-tight interface between the transfer tube and the output end of the light pipe.
A further object of the present invention is to simplify the attachment of the light pipe to the transfer tubes by substantially eliminating the use of sealants and by minimizing the number of fittings without affecting high chromatographic resolution and spectrometric detection due to interaction of the sample with the seals and fittings.
An object of the invention is to eliminate unswept areas or dead spaces in the flow path of the GC/IR interface assembly.
An object of the invention is to simplify the heating of the light pipe and transfer tubes while at the same time eliminating cold spots and facilitating the adaptation of the light pipe and transfer tubes for use in a variety of FTIR sample chambers.
Another object of the invention is to provide optimal temperature control of the light pipe and transfer tubes while at the same time facilitating the adaptation of the light pipe and transfer tubes for use in a variety of FTIR sample chambers.
A further object of the invention is to improve upon the chemical inertness of the fittings and transfer tubes in order to provide accurate sample detection.